[WIP] Fix integer overflow in ArrayList BinarySearch

[Copilot]

• Understand the issue: Integer overflow in IListWrapper.BinarySearch when calculating mid = (lo + hi) / 2 with large indices
• Find the correct fix pattern: Use lo + ((hi - lo) >> 1) as done in Array.GetMedian
• Run baseline build (clr+libs -rc release)
• Make the fix to ArrayList.cs line 774
• Add a unit test to validate the fix
• Run tests to verify the fix works
• Run code review and codeql checker

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ArrayList IListWrapper integer overflow in BinarySearch</issue_title>
<issue_description>Consider the following program:

using System;
using System.Collections;

static class Foo
{    
    static void Main()
    {
        IList l = new byte[int.MaxValue / 2 + 2];
        ArrayList al = ArrayList.Adapter(l);
        int result = al.BinarySearch((byte)1);
        Console.WriteLine(result);
    }
}

This will overflow here:

runtime/src/libraries/System.Private.CoreLib/src/System/Collections/ArrayList.cs

Line 774 in be04169

mid = (lo + hi) / 2;

I would instead expect this to return a value indicating it could not be found.

The GetMedian method on Array is likely the "right" implementation to use for calculating mid.</issue_description>

<agent_instructions>ArrayList is in corelib, so after doing a full build, for any subsequent changes you make, you'll need to rebuild corelib with clr.corlib+clr.nativecorelib+libs.pretest.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes ArrayList IListWrapper integer overflow in BinarySearch #123804

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[vcsjones]

@stephentoub I had a Copilot session going here (unfortunately the way I initiated it resulted in it not being assigned to Copilot) #123806